It is well established that surgical patients under anesthesia become poikilothermic. This means that the patients lose their ability to control their body temperature and will take on or lose heat depending on the temperature of the environment. Since modem operating rooms are all air conditioned to a relatively low temperature for surgeon comfort, the majority of patients undergoing general anesthesia will lose heat and become clinically hypothermic if not warmed.
Over the past 15 years, forced-air warming (FAW) has become the “standard of care” for preventing and treating the hypothermia caused by anesthesia and surgery. FAW consists of a large heater/blower attached by a hose to an inflatable air blanket. The warm air is distributed over the patient within the chambers of the blanket and then is exhausted onto the patient through holes in the bottom surface of the blanket.
Although FAW is clinically effective, it suffers from several problems including: a relatively high price; air blowing in the operating room, which can be noisy and can potentially contaminate the surgical field; and bulkiness, which, at times, may obscure the view of the surgeon. Moreover, the low specific heat of air and the rapid loss of heat from air require that the temperature of the air, as it leaves the hose, be dangerously high—in some products as high as 45° C. This poses significant dangers for the patient. Second and third degree burns have occurred both because of contact between the hose and the patient's skin, and by blowing hot air directly from the hose onto the skin without connecting a blanket to the hose. This condition is common enough to have its own name—“hosing.” The manufacturers of forced air warming equipment actively warn their users against hosing and the risks it poses to the patient.
Electric warming blankets overcome the aforementioned problems with FAW. Some of these warming blankets employ flexible heaters, the flexibility of which is desirable to maintain when employing in the blankets. In applications such as these, where the heater is subject to flexing, couplings directly between the heater and bus bars, which extend along opposing edges of the heater to supply power to the heater, may be susceptible to zones of intermittent contact along a length of each of the bus bars. Thus there is a need for flexible heater subassemblies wherein interfaces between bus bars and heaters are augmented to facilitate more uniform contact therebetween.
The standard method of coupling the electrical power supply to any large heater surface is to place a metal bus bar conductor near two of the opposing edges of the heater. Electrical power flows from the power supply through the bus bars and is evenly distributed along the entire length of the heater. The electrically conductive bus bar material contacts the electrically conductive heater and the current flows between the two materials. Unfortunately, the conductive bus bars do not make a dependable, uniform and stable connection to the conductive heater, especially during flexing of the heater, because both the heater and the bus bar are flexible. Generally, two flexible pieces of material that are placed together will not maintain reliable and uniform contact across their entire surface, especially during repeated flexing.
When the bus bar/heater interface is flexed, the heater temporarily separates slightly from the bus bar at point locations. This separation prevents current from flowing at the separation point, forcing the current that would have passed through that point to flow instead through adjacent points that are still in contact. The increased current flowing through the adjacent points can cause those points to over-heat. Repeated over-heating can cause the heater at that point to eventually fail and stop conducting electricity. When a point fails, it is permanently removed from the current path and the adjacent points must pick up the extra flow. The extra flow caused by the failed points, in addition to the extra flow caused by the areas of non-contact due to flexion, may result in over-heating and failure of the remaining points.
Accordingly, there remains a need for flexible heater subassemblies and blankets that allow the bus bar and the heater to be coupled in such a manner that current can be dependably and uniformly supplied from the bus bar to the heater without potentially patient harming blanket over-heating and/or failure. Various embodiments of the invention described herein solve one or more of the problems discussed above.